This invention is concerned with a process for manufacturing a self-supporting filter unit having a plurality of passages defined by air-permeable wall portions through which air to be filtered can pass and be drawn off along said passages but which prevent the passage therethrough of dust and/or other debris carried by such air.
Known dust filter units are in the form of bags which are supported against collapse by a framework of metal or other suitable material, air to be filtered being sucked through the bags from outside to the interior thereof so that dust particles and/or other debris are prevented by the material of the base from passing to the interior and thus collected on the outer surface; the pore size of the fabric of these filter bags is selected according to the size of dust particles to be filtered from the gas flow. Self-supporting filter units, i.e. units which do not require a framework of metal or other suitable material, are available; one such unit is supplied under the trademark "SINTAMATIC" by DCE Limited of Thurmaston, Leicester. This filter unit comprises a porous composite which is rigid and self-supporting, being made by a process in which a blend of granulated engineering polymers is dimensionally restrained and processed to produce a rigid porous composite element. The surface of this element is then treated with a P.T.F.E.-based solution to form a microporous skin. While this self-supporting filter has been shown to be generally satisfactory in performance, it is somewhat expensive to provide and the production process is relatively complex.